Remove QWidget::setWindowHandle

It's unused. The window (QWidgetWindow) is created and managed implicitly
by QWidget.

Change-Id: I28ee3f120a99c877f318e1abd7d73c9f3e542d03
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/widgets/kernel/qwidget.h

@@ -607,7 +607,6 @@ public:
 
     QBackingStore *backingStore() const;
 
-    void setWindowHandle(QWindow *window);
     QWindow *windowHandle() const;
 
     friend class QDesktopScreenWidget;